NUTRIENT CONCENTRATIONS IN WASTEWATER-TREATMENT-PLANT EFFLUENTS, SOUTH PLATTE RIVER BASIN

National Water-Quality Assessment (NAWQA) Program
South Platte River Basin Study

by Matthew Pocernich 1and David W. Litke 2

ABSTRACT

Accurate data about nutrient concentrations in wastewater-treatment-plant
effluents are needed for river basin water-quality studies. As part of the U.S. Geological Survey's
National Water-Quality Assessment Program in the South Platte River Basin, nutrient data were
requested from 31 wastewater-treatment plants located in the basin. This article describes
the types of nutrient data available from the plants, examines the variability of effluent
nutrient concentrations, and discusses methods for estimation of nutrient concentrations
where data are lacking.

Ammonia was monitored at
88 percent of the plants, nitrite plus nitrate was monitored at 40 percent of the plants, and
organic nitrogen and phosphorus data were monitored at less than 25 percent of the plants, and
organic nitrogen and phosphorous were monitored at less than 25 percent of the plants. Median
total nitrogen concentrations and median total phosphorus concentrations were small compared to
typical literature estimates for wastewater-treatment plants with secondary treatment. Nutrient
concentrations in effluent from wastewater-treatment plants varied widely between and within plants.
For example, ammonia concentrations varied as much as 5 mg/L during a day, as much as
10 mg/L from day to day, and as much as 30 mg/L from summer to winter within a plant.
In the South Platte River Basin, estimates of median annual ammonia and nitrite plus
nitrate concentrations can be improved based on plant processes; and nitrite plus
nitrate and organic nitrogen concentrations can be estimated based on ammonia
concentrations. However, to avoid large estimation errors, more complete nutrient data
from wastewater-treatment plants are needed for integration into river basin water-quality
studies. The paucity of data hinders attempts to evaluate the relative importance of
point source and nonpoint source nutrient loadings to rivers.